Accurate measurements of the displacements of moving devices are often required. However, the desired precision is often difficult to obtain, chiefly when very limited angular displacements are to be measured.
At present, the so-called optical encoders are the most accurate angle measuring systems. They generally consist of a glass disk, whose surface has suitably arranged opaque and transparent sectors and which is integral with a rotating shaft, the angular position of which is to be determined.
The disk is interposed between a light source and a photodetector which, according to whether the encoder is an incremental or an absolute encoder, supplies either a train of pulses, whose number is proportional to the increment of the angle by which the shaft is rotated, or a bit configuration coding the shaft position.
These systems allow angles of the order of the seconds of degree to be measured, yet they have mechanical sensitivity and precision limits due, on the one hand, to the impossibility of reducing beyond a certain limit the sizes of the opaque and transparent sectors and of the detector sensing surface and, on the other hand, to the need far limiting the encoder disk size. Moreover, the precision decreases as the angle to be measured decreases.